Update timer if no jobs found by mongo worker.

I noticed that keeping a mongo-worker running without submitting a job for a long time causes timeouts in `run_one()`. I think the cause is this logic:

```python
       start_time = time.time()
        mj = self.mj
        while job is None:
            if (reserve_timeout
                    and (time.time() - start_time) > reserve_timeout):
                raise ReserveTimeout()
            job = mj.reserve(host_id, exp_key=self.exp_key)
            if not job:
                interval = (1 +
                        numpy.random.rand()
                        * (float(self.poll_interval) - 1.0))
                logger.info('no job found, sleeping for %.1fs' % interval)
                time.sleep(interval)
```

Essentially if there are no jobs submitted the timer just keeps on running until it time-outs. Resetting the timer changes this behavior.